Electrically small resonant antenna with capacitively coupled load

ABSTRACT

An antenna having dimensions as small as 0.03 wavelength of the wavelength to be received comprising a helix wire having a loading disc attached to one end and an RF connector attached to the other. The connector shield is mounted on a grounded disc and a conductive member is electrically associated with, but spaced from the loading disc.

llnited States Patent [111 3,852,756

Reese Dec. 3, 1974 ELECTRICALLY SMALL RESONANT ANTENNA WITH CAPACITIVELYCOUPLED LOAD Joe Reese, China Lake, Calif.

Assignee: The United States of America as represented by the Secretaryof the Navy, Washington, DC.

Filed: Feb. 15, 1974 Appl. No.: 442,933

Inventor:

US. Cl 343/708, 343/807, 343/895 Int. Cl. H01q 1/36 Field of Search343/705, 708, 752, 807,

References Cited UNITED STATES PATENTS 4/l97l Gouillou et al. 343/895Primary E,\'nminer-Eli Lieberman Attorney, Agent, or Firm-R. S.Sciascia; R. Miller; R. W. Adams [57] ABSTRACT An antenna havingdimensions as small as 0.03 wavelength of the wavelength to be receivedcomprising a helix wire having a loading disc attached to one end and anRF connector attached to the other. The connector shield is mounted on agrounded disc and a conductive member is electrically associated with,but spaced from the loading disc.

10 Claims, 2 Drawing Figures Pmmmw W iasmss sum 10? 2 Fig. l

PAIENIMEB 31m 38%?56 SHEH 2 OF 2 FIG. 2

ELECTRICALLY SMALL RESONANT ANTENNA WITH CAPACITIVELY COUPLED LOADBACKGROUND OF THE INVENTION In the antenna field, there are manypossible applications, and a pressing need, for electrically smallantennas. Standard antenna elements become essentially inoperative whentheir dimensions become less than 0.1 wavelengths. That is, when thedimensions of the antenna become less than l/ the wavelength tobereceived, or sent, they become highly inefficient. The presentinvention, on the other hand, provides efficient operation withdimensions as small as 0.03 wavelengths.

The present invention may be used anywhere a small antenna is desired orrequired, such as in the restricted volume of the nose cone of a missileor projectile. The antenna may be designed for any frequency, but isespecially beneficial at the lower frequencies where stan- 1 dardantenna elements are relatively large.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. l is a plan view of a firstembodiment of the present invention; and

FIG. 2 is a plan view of a second embodiment of the present invention.

DESCRIPTION" OF THEPREFERRED EMBODIMENT The first preferred embodimentof the present inven tion is shown in FIG. 1. The embodiment is shownincorporated in the nose cone of a'missile 10, as an example of onepossible application of the present invention. The embodiment comprisesa wire helix 12 as an extension of the inner conductor of coaxial cable14. The free end of helix 12 terminates in a loading disc 16 which maybe in the shape of a circular disc, a square or rectangular plate, acone as shown in the second embodiment, or in any other shape designedfor the intended application, including the wheel and spoke design shownin US Pat. No. 3,573,840 entitled Small Bulk Helically Wound AntennaeAnd Method For Making Same" by R. L. Gouillou et al.

Helix l2 and loading disc 16 are electrically associated with cone 18which is attached, such as by plating, to the exterior surface to nosecone 20. Nose cone 20 may be the nose cone designed for the missile 10,or may be specially designed to incorporate conductive cone 18. In mostapplications, nose cone 20 will be Teflon, some form of low lossplastic, or other material chosen for its ability to transmit thefrequency of inter est. Cone 18 is electrically associated with helix l2and loading disc 16, as mentioned above, but is not physically connectedthereto, although it is an integral part of the antenna.

The other conductor of coaxial cable 14 is connected to ground plate 22.When the antenna is employed in a missile, the body of missile 10, whichis usually metallic and conductive, may be coupled to ground plate 22 toprovide an increase in the apparent ground plate for the antenna.

Electrically, the antenna resembles a dipole wherein the outer conductorof the coaxial cable is connected to one element of the dipolecomprising the shell of missile l0 and ground plate 22. The innerconductor of coaxial cable 14 is connected to the other element of thedipole which in this case comprises helix 12, load-.

ing disc 16 and conductive cone 18. In a dipole antenna the elements arechosen for the wavelength of interest, and must be equal in electricallength as seen at their terminals for optimum operation. This inventionthough assymetrical in configuration greatly improves the ratio of thedipole elements by increasing the electrical length of the short (helix)side.

If prior techniques were utilized in an attempt to provide a small,resonant, helical antenna, the simulated dipole would fail to radiatesignificantly. The failure results from the antennas inability to set upsufficient resonant currents in the radiating elements. The antennawould also fail as a receiving antenna for the same reason, i.e., theantennas insufficient electrical length to develop and maintain aresonant current of significant value at the frequency of interest.

All electrically small antennas characteristically have a high value ofcapacitive reactance. A helical antenna is a series of turns of wirethat adds inductance to the circuit when the antenna is exposed toalternating currents, such as are employed in transmitting andreceiving. By adding capacitance to the circuit, it becomes LC tunableto achieve a resonant condition. In the present invention, the top hat16 connected to the free end of helix 12 decreases the capacitivereactance of the antenna and permits radio frequency coupling toconductive cone 18 on the exterior of dome 20. Thereby, the resonanttuned circuit is able to excite the entire shell body of the missile 10,which becomes the radiating antenna. The radiating pattern of theequivalent dipole has its maximum beam broadside to the shell, withnulls along the shell axis.

FIG. 2 shows an embodiment that is designed for a space even smallerthan that intended for the embodiment of FIG. 1. The further reductionin size is achieved by additionally decreasing the capacitive reactance.As before, the interior conductor of coaxial cable 14 is directlyconnected to helix 12 which terminates in top hat 16. Although loadingdisc 16 is normally flat it may be shaped to fit the container in whichit is to be placed to further reduce the volume filled by the antenna.As before, conductive cone 18 on dome 20 is electrically coupled tohelix l2 and loading disc 16. The outer conductor of the coaxial cable14 is connected through ground plate 22 to the shell of the container10.

The capacitive reactance is additionally reduced by including dielectriccylinder 24 around helix 12. As the capacitive reactance is decreasedthe physical length of helix l2 necessary to attain resonance is alsodecreased. The dielectric cylinder, which should be of a high Kmaterial, decreases the capacitive reactance /zrrjC where C iscapacitance) by increasing the capcacitance appearing in the circuit.The effect is to increase the effective length of helix 12. By reducingthe capacitive reactance, the original antenna may be operated at alower frequency. Or, a smaller helix 12 may be used for operation at thesame frequency. The addition of the dielectric does, however, absorbsome of the power which reduces the antenna gain.

A primary advantage of the present invention over the prior devices isthat antennas having dimensions less that 1] l0 wavelength areattainable. For other frequencies the antenna is changed by simplyscaling the dimensions of the invention for the wavelength of interest.In addition to the possible reduction of size, another advantage isa'substantial increase in gain attainable. The combination of theresonant helix and RF coupling to the external cone has demonstratedgain increases of from 5 to dB over other configurations of equal size.

What is claimed is:

1. A physically small resonant antenna designed for utilization inapplications having a relatively small volume in which to position anantenna for the wavelength to be transmitted or received, comprising:

an electrically conductive helical member having first and secondopposite ends;

means for reducing the capacitive reactance of said antenna, includingfirst and second electrically conductive members wherein said membersare physically spaced apart and the first member is coupled near thefirst end of said helical member;

circuit means for processing an electrical signal, having at least firstand second terminals;

a coaxial member having a plurality of electrically conductive elementswherein the first element couples said circuit means to the second endof said helical member; and

an electrically conductive enclosure containing said circuit means andsaid coaxial member, wherein the second terminal of said circuit meansis coupled by the second element of said coaxial member to saidenclosure;

such that said antenna is a dipole antenna having first and second armswherein the apparent electrical length of the first 'arm sufficient lyapproximates the electrical length of the second arm so that saidantenna operates as a dipole antenna for the chosen frequency ofinterest.

2. The antenna of claim 1 wherein the first member of said reactancereducing means is an element having a circular profile which is attachedat its center to said helical member.

3. The antenna of claim 2 wherein said enclosure is a missile bodyincluding a ground plate to which said coaxial member is attached andsaid second element of said coaxial member is electrically connected,and through which said first element of said coaxial member passes tosaid helical member, wherein said missile body also includes anelectrically non-conductive shroud in covering relationship to saidhelical member and the first member of said reactance reducing means.

4. The antenna of claim 3 wherein the second member of said reactancereducing means is on the outer surface of said shroud with respect tothe first member of said reducing means.

5. The antenna of claim 4 wherein said coaxial member is a coaxial cablehaving its inner conductor connected to said helical member and itsouter conductor connected to said ground plate.

6. The antenna of claim 5 including an annual member having a highdielectric constant surrounding said helical member.

7. The antenna of claim 6 wherein the first member of said reducingmeans is a cone.

8. The antenna of claim 6 wherein the first member of said reducingmeans is a disc.

9. The antenna of claim 6 wherein the first member of said reducingmeans is a plurality of arms radiating at right angles to the helicalmember, connected to one another at their outermost extremity by acircular ring.

10. The antenna of claim 6 wherein the length of said antenna is lessthan one-tenth the wavelength of the electrical signal, and the antennaand circuit means form a radar.

1. A physically small resonant antenna designed for utilization inapplications having a relatively small volume in which to position anantenna for the wavelength to be transmitted or received, comprising: anelectRically conductive helical member having first and second oppositeends; means for reducing the capacitive reactance of said antenna,including first and second electrically conductive members wherein saidmembers are physically spaced apart and the first member is coupled nearthe first end of said helical member; circuit means for processing anelectrical signal, having at least first and second terminals; a coaxialmember having a plurality of electrically conductive elements whereinthe first element couples said circuit means to the second end of saidhelical member; and an electrically conductive enclosure containing saidcircuit means and said coaxial member, wherein the second terminal ofsaid circuit means is coupled by the second element of said coaxialmember to said enclosure; such that said antenna is a dipole antennahaving first and second arms wherein the apparent electrical length ofthe first arm sufficiently approximates the electrical length of thesecond arm so that said antenna operates as a dipole antenna for thechosen frequency of interest.
 2. The antenna of claim 1 wherein thefirst member of said reactance reducing means is an element having acircular profile which is attached at its center to said helical member.3. The antenna of claim 2 wherein said enclosure is a missile bodyincluding a ground plate to which said coaxial member is attached andsaid second element of said coaxial member is electrically connected,and through which said first element of said coaxial member passes tosaid helical member, wherein said missile body also includes anelectrically non-conductive shroud in covering relationship to saidhelical member and the first member of said reactance reducing means. 4.The antenna of claim 3 wherein the second member of said reactancereducing means is on the outer surface of said shroud with respect tothe first member of said reducing means.
 5. The antenna of claim 4wherein said coaxial member is a coaxial cable having its innerconductor connected to said helical member and its outer conductorconnected to said ground plate.
 6. The antenna of claim 5 including anannual member having a high dielectric constant surrounding said helicalmember.
 7. The antenna of claim 6 wherein the first member of saidreducing means is a cone.
 8. The antenna of claim 6 wherein the firstmember of said reducing means is a disc.
 9. The antenna of claim 6wherein the first member of said reducing means is a plurality of armsradiating at right angles to the helical member, connected to oneanother at their outermost extremity by a circular ring.
 10. The antennaof claim 6 wherein the length of said antenna is less than one-tenth thewavelength of the electrical signal, and the antenna and circuit meansform a radar.